International Journal of Mechanical Sciences最新文献

Acoustic metasurfaces (AMs) can manipulate acoustic waves in ways that are not reachable in natural materials, offering significant implications for engineering applications such as noise control. While previous studies have primarily been conducted in stationary mediums, this study delves into the sound reflections of wave-manipulation AMs subjected to a grazing flow. An analytical model is developed to predict the sound reflections of both periodic and non-periodic AMs under flow conditions based on the plane-wave expansion. The flow effects on the periodic and focusing AMs are analytically and numerically investigated. Experiments are also conducted in a newly designed aeroacoustic oblique plane wave (AOPW) facility at the Hong Kong University of Science and Technology (HKUST). Results show that the reflected sound pressure fields of wave-manipulation AMs under flow conditions can be predicted well by the analytical model. Good absorption of the periodic AMs can be achieved by adjusting the periodic length-to-wavelength ratio to below $(1-M_0^2)/2$ due to the surface wave conversion mechanism. The focal points of the AMs designed in the stationary air shift to the downstream direction due to the flow effects, which can be corrected by the proposed analytical model. The focusing AM design is also extended into a three-dimensional (3D) space and is validated analytically. This study extends the understanding of wave-manipulation AMs into flow conditions, which may help the AM design operating in non-stationary mediums, such as air and water flows.

A new approach to scaled experimentation has appeared in the open literature bringing into existence a countably infinite number of similitude rules connecting multiple scaled experiments. The simplest rule (the zeroth-order rule) captures all what is possible with dimensional analysis but higher-order rules appear to necessitate investigations at multiple scales. The scaling theory finite similitude can however, be repurposed for the analysis of scaled models making it possible to relate models of two different sizes whilst automatically accounting for all scale effects present. The new approach to scaling analysis gives rise to additional systems of equations that are required to be solved and it is this aspect that is the main focus of this paper. It is shown through application of the new scaling-analysis approach to mechanical systems built from discrete elements (e.g., springs, lumped masses, dampers) how scale effects are directly represented. Scaling analysis under the finite-similitude framework is shown to be effective for connecting up scaled models but additionally dovetails with experimental approaches involving scaled experiments. Through application to mechanical systems the new formulation is shown to have practical value but also reveals how system-level scale effects can be handled efficiently. The approach provides a framework for the design and analysis of mechanical components that are required to operate over a range of sizes.

The development of structures capable of both dynamic shape morphing and stiffness modulation has significant potential in various applications. However, such structures often suffer from bulkiness and control complexity. This paper addresses these challenges by exploring a scaled structure that integrates morphing capabilities and variable stiffness within a compact configuration. For the first time, we establish a comprehensive set of design criteria and obtain the previously unexplored design space, focusing on geometric parameters including layer thickness, target shape radius, the number of scales, and the number of periods per scale. Through extensive finite element simulations, we evaluate the impact of material property and geometric parameters on the performance of the scaled structure, emphasizing the role of coefficient of friction. Our findings identify a critical threshold for the coefficient of friction above which morphing ability is hindered. Additionally, we uncover a trade-off between morphing capability and stiffness variation ability, which we overcome by modifying the surface structure of the scales. The optimal design is found to be a superellipse shape with an exponent of ∼1.9. The practical potential of this structure is demonstrated through three applications: a soft gripper, a phone stand, and a foldable box, showcasing its versatility in real-world scenarios. This research provides a foundational approach for designing morphing scaled structures, offering valuable insights into optimizing morphing capability and stiffness variation ability for broader engineering applications.

Surface morphological patterns are widely observed in natural systems, which are closely correlated to vital biological functions and inspire surface morphology designs in soft matter systems. Geometrical incompatibility widely exists in biological tissues across different length scales and plays an important role in growth-induced pattern selection and morphological evolution of soft tissues. However, the underlying physical mechanism of growth-induced pattern formation and post-buckling evolution in geometrically incompatible spherical soft tissues remain elusive. Here, the effect of geometrical incompatibility on the growth-induced pattern selection and post-buckling evolution are investigated through swelling experiment, theoretical analysis and numerical simulation. The results show that not only the instability pattern but also the instability threshold can be regulated by manipulating geometric incompatibility. Notably, when the geometrical incompatibility parameter exceeds a critical value, spontaneous instability is observed before growth. With continuous growth, the core–shell soft sphere buckles into a periodic buckyball pattern and evolves toward a bean-shaped pattern, and then undergoes a wrinkle-to-fold transition into a labyrinth topography. Our results demonstrate, both experimentally and theoretically, that geometrical incompatibility can guide the growth-induced pattern formation and morphological evolution effectively. This study not only enhances our understanding of the growth-induced pattern selection and morphological evolution in spherical soft tissues, but also provides an inspiring insight for the fabrication of morphological patterns on curved surfaces.

The multiple sub-impact phenomenon, consisting of more than one short contact event, has frequently been observed in experiments and simulations. The multiple sub-impact phenomenon and its influences on impact responses lack systematic study. Hence, this paper presents a parametric analysis method, an extended hybrid, numerical-analytical model (EHNA model). The nature of multiple sub-impact responses of elastic-plastic beams struck by elastic-plastic spherical impactors is investigated parametrically. The occurrence and disappearance of multiple sub-impacts are observed. Their threshold curves are obtained and expressed analytically for low-velocity impacts. A characterization diagram is proposed to characterize the region of multiple sub-impacts in the relative stiffness ratio-effective mass ratio plane. The characterization diagram can be used to quickly predict the state of multiple sub-impacts or the state of a single impact without solving the impact responses in detail. It is validated experimentally and numerically for impacts with wide ranges of initial velocity, beam size, constraint, material property and contact type. The influences of multiple sub-impacts on impact force response, impact impulse, and energy loss are parametrically investigated. The high influence zones and several influence features are observed. The wide multiple sub-impact region proves that the multiple sub-impact phenomenon is ubiquitous. The strong influences on flexible beam impacts indicate that multiple sub-impacts cannot be neglected for structural damage and structural dynamics.

Among the various methods for strain sensing, the metamaterial absorbers (MMAs) stand out due to their dual capabilities. Specifically, MMAs facilitate the wireless detection of deformations in the target and operate independently of any external power source. However, conventional research has a limitation in that stretchable strain sensors are unable to deform themselves autonomously, which puts constraints on being efficiently utilised in special environments where human intervention is difficult. Herein, we propose a wireless, power-independent, biaxial strain sensor equipped with self-shape and frequency recovery capability that addresses the limitations of existing wireless strain sensors through the unprecedented integration of a 4D-printed shape memory actuator and a biaxially stretchable MMA. The novel integration with the shape memory actuator enables the stretchable MMA to autonomously recover to its original shape and absorption frequency after being heated to 70 °C for a few minutes. This smart functionality enables the resulting wireless strain sensor based on the proposed idea to revert to the original state when sensing a new target without requiring human intervention. The highly sensitive biaxial sensing capability is as follows. When stretched horizontally from 0 % to 30 %, the absorption frequency of the proposed biaxially stretchable MMA demonstrates a linear change from 9.75 GHz to 7.94 GHz, exhibiting a high sensitivity of 4.3 × 10^7 Hz/%. Similarly, when stretched vertically from 0 % to 30 %, the absorption frequency linearly changes from 7.35 GHz to 6.01 GHz, indicating a sensitivity of 5.9 × 10^7 Hz/%. Accordingly, the wireless biaxial sensing capability of the proposed stretchable MMA, as well as its shape-recovery functionality facilitated by the 4D-printed actuator are highly effective for remote strain measurement in environments where direct human involvement is impractical.

Moving components exposed to electron radiation over longer durations are more prone to failure due to its complex changes from material properties to component characteristics. It involves multi-scale analysis, leading to current methods being deficient in accuracy and efficiency. In this paper, a cross-level vibration prediction method, which selects the ultrasonic motor (USM) stator as a typical component for Jupiter exploration, is proposed by incorporating the cross-scale changes of material properties based on the edge-based smoothed finite element method (ES-FEM). A cross-scale degradation model for exploring the material properties is constructed by establishing the correlation between the degradation of molecular chains and the mechanical properties of the epoxy resin. The ES-FEM is developed for investigating the vibration of the USM stator, by introducing the edge-based gradient smoothing technique (GST) to perform the strain smoothing operation in its stiffness matrix, offering superior accuracy and efficiency. The experiment of 1.2 MeV electron radiation under different electron fluences was carried out. It demonstrated that the present method can achieve higher accuracy and efficiency than the traditional one, while being closed to the experimental results with the frequency and amplitude errors of 0.03 % and 1.3 %, respectively.

Rigid foldable origami enables smooth and precise folding without stretching or bending its constituent panels and is promising for applications such as reprogrammable matter, self-folding machines, reconfigurable antennas, and deployable spacecraft. The diverse range of potential applications necessitates the need for the design and detailed analysis of different rigid-foldable origami structures, especially those with intricate motion modes. In this paper, we introduce a rigid-foldable spiral origami design that features a compression-torsion coupled motion mode. This design exhibits rich static and dynamic properties. Under static conditions, the compression-torsion coupled motion mode creates multiple self-locking positions and allows for the development of mechanical static diodes. Under dynamic conditions, the compression-torsion coupling effect in the spiral origami facilitates precise control of wave modes within the origami chain when impacted by a ball with a moderate initial velocity. In the case of large initial velocities of the ball, the spiral origami can function as a wave generator, producing rarefaction solitary waves or compressive solitary waves. The proposed spiral origami design provides an opportunity to explore new applications of rigid-foldable origami with compression-torsion coupling effects.

Shrinkage-induced cracking significantly impacts the durability of mass concrete structures. Quantitatively evaluating drying shrinkage of concrete proves challenging due to the time-consuming experiments and overlooked microstructure changes during the hydration process. To address this concern, this study initially characterized the long-term hydration products and microstructure of low-heat Portland cement (LHPC) through microstructural experiments. Subsequently, a novel high-resolution mesoscale framework is developed to investigate the drying shrinkage with hydration kinetics. High-resolution models consist of realistic-shaped aggregates are validated by the aggregate morphology and gradation parameters of core sample from mass concrete. Concurrently, the quantitative effects of internal and external factors on LHPC drying shrinkage are explored. Results indicated that LHPC possesses a denser microstructure, lower porosity, higher carbonation resistance, and 20% lower drying shrinkage compared to moderate-heat Portland cement, suggesting promising applications. Furthermore, experimental and computational findings suggested that increasing aggregate volume, controlling aggregate morphology, and adjusting curing time and humidity could be employed to reduce and manage drying shrinkage, ensuring concrete structure durability.